1. Field of the Invention
The present invention relates to an impeller having blades with a swept-forward shape and arranged to produce an airflow along a central axis thereof, about which the impeller rotates, a fan apparatus using this impeller, and a method of manufacturing the impeller.
2. Description of the Related Art
Electronic devices such as personal computers or servers are often provided with a cooling fan to cool electronic components within a case thereof. There is a demand for an improvement in the performance of the cooling fan with increasing density of the electronic components within the case. In general, the cooling fans can be divided into two types: exhaust fans designed to expel hot air within the case from the case; and blower fans designed to send cooling air directly to the heated electronic components. In the blower fans, the direction of the airflow, i.e., the direction in which the air is sent, is important.
In a case where the cooling air is sent directly to the heated electronic components, it is desirable that a great quantity of the airflow directed to the electronic components, and the blower fans are suitable for this use.
Impellers having blades with a swept-forward shape are often used in blower fans because the swept-forward blades have a characteristic of preventing the airflow from expanding radially outward when producing the airflow. The term “swept-forward shape” as used herein refers to a blade shape in which a straight line joining the central axis and a point of intersection of a leading edge of each blade with a radially outer end of the blade is positioned forward, with respect to a rotation direction, relative to a point of intersection of the leading edge of the blade with a base of the blade. For example, an impeller used in an axial blower described in JP-A 2008-196480 has blades with the swept-forward shape. The degree to which the aforementioned straight line is positioned forward relative to the aforementioned point of intersection varies.
Rotation of the impeller causes a centrifugal force to be applied to the blades. The centrifugal force is directed in a direction substantially parallel to a radial direction, from the base of each blade where the blade is joined to a support portion. Impellers that are to be rotated at a high speed need to be designed to have sufficient strength, with consideration given to stress imposed on the base of each blade due to the centrifugal force. This influence of the centrifugal force becomes more significant as the speed at which the impeller rotates increases.
The influence of the centrifugal force is great with impellers having blades in which the degree of the forward sweep of the blades is great. Since the radially outer end of each blade of such an impeller is positioned forward relative to the base of the blade with respect to the rotation direction, the centrifugal force produced on entire portions of the blade produces a large moment at the base of the blade. This moment produced at the base of each blade needs be taken into consideration when the impeller is designed. The strength of each blade is lowest at its base because stress concentration occurs at the base of the blade.
Moreover, the aforementioned influence of the centrifugal force may deform the blades so that the radially outer ends of the blades may be displaced radially outward. The radially outward displacement of the radially outer ends of the blades may cause the radially outer ends of the blades to make contact with an inner surface of an outer frame member surrounding the impeller.
As effective countermeasures against the aforementioned influence of the centrifugal force, U.S. D511824, U.S. 2008/0056899, and U.S. Pat. No. 6,554,574 disclose substantially annular joining members arranged to join the blades to one another. In impellers provided with such a joining member, a region radially inward of the joining member joining the blades to one another greatly affects an air flow quantity characteristic of the impeller, whereas a region radially outward of the joining member greatly affects a surge characteristic and a static pressure characteristic of the impeller.
Furthermore, in axial fans, a backward airflow occurs at a gap defined between an inner surface of an outer frame member containing the impeller and the radially outer ends of the blades. This phenomenon manifests itself most strikingly in a surge range which leads to a deterioration in the static pressure characteristic and increased noise levels in the surge range. This problem manifests itself noticeably in cases where the joining member is provided at the radially outer ends of the blades. It is noted that the impeller described in U.S. 2008/0056899 uses sweptback blades, and is different in structure from impellers according to preferred embodiments of the present invention described below.